Diseases with the onset of deposition of a fibrous protein called amyloid in various organs or tissues in bodies are generally referred to as amyloidosis. A feature common to amyloidosis is that the fibrous protein called amyloid which is enriched with the β-sheet structure is deposited at various organs systemically or at sites topically so that functional abnormalities are triggered in the organs or tissues.
Alzheimer's disease (hereinafter referred to as AD), which is a typical amyloidosis disease, is known as a disease causing dementia. This disease is lethal with progressive deposition of amyloid in brain, and thus is said to be a disease that causes concern in society compared with other amyloidosis diseases. In recent years, the number of AD patients is rapidly increasing in developed countries with aging societies, thereby causing a social problem.
From the pathohistological viewpoint, AD is characterized by three pathological findings in brain, namely development of senile plaques, formation of neurofibrillary tangles, and extensive neuronal loss. The senile plaque has a structure composed mainly of amyloid, and is said to appear at the earliest stage of AD onset and thus is pathologically found in brain about 10 or more years before appearance of clinical symptoms.
AD is diagnosed by carrying out various evaluations of cognitive functions (for example, Hasegawa scale, ADAS-JCog and MMSE) in auxiliary combination with imaging diagnosis such as CT and MRI. However, the method based on such evaluations of cognitive functions is low in diagnostic sensitivity at the early stage of the onset, and is furthermore problematic in that diagnostic results are susceptible to inborn cognitive functions of individuals. At present, it is practically impossible to establish a definite diagnosis of AD while an AD patient is still alive, because the definite diagnosis requires a biopsy of a lesion (Non-Patent Document 1).
Meanwhile, a report tells that amyloid constituting senile plaques is an aggregate of amyloid β protein (hereinafter referred to as Aβ). Also, numerous reports tell that the Aβ aggregate forms a β-sheet structure that causes nerve cell toxicity. Based on these findings, the so-called “Amyloid Cascade Hypothesis” is proposed, which suggests that cerebral deposition of Aβ triggers the downstream phenomena, namely, formation of neurofibrillary tangles and neuronal loss (Non-Patent Document 2).
Based on these facts, attempts have recently been made to detect AD in vivo using a compound having high affinity with amyloid as a marker.
Many of such probes for imaging diagnoses of cerebral amyloid are hydrophobic low-molecular compounds that are high in affinity with amyloid and high in cerebral transferability and are labeled with various radioactive species such as 11C, 18F and 123I. For example, reports tell 11C or radioactive halogen labeled forms of compounds including various thioflavin derivatives such as 6-iodo-2-[4′-(N,N-dimethylamino)phenyl]benzothiazole (hereinafter referred to as TZDM) and 6-hydroxy-2-[4′-(N-methylamino)phenyl]benzothiazole (hereinafter referred to as 6-OH-BTA-1) (Patent Document 1, Non-Patent Document 3); stilbene compounds such as (E)-4-methylamino-4′-hydroxystilbene (hereinafter referred to as SB-13) and (E)-4-dimethylamino-4′-iodostilbene (hereinafter referred to as m-I-SB) (Patent Document 2, Non-Patent Document 4, Non-Patent Document 5); benzoxazole derivatives such as 6-iodo-2-[4′-(N,N-dimethylamino)phenyl]benzoxazole (hereinafter referred to as IBOX) and 6-[2-(fluoro)ethoxy]-2-[2-(2-dimethylaminothiazol-5-yl)ethenyl]benzoxazole (Non-Patent Document 6, Non-Patent Document 7), DDNP derivatives such as 2-(1-{6-[(2-fluoroethyl)(methyl)amino]-2-naphthyl}ethylidene)malononitrile (hereinafter referred to as FDDNP) (Patent Document 4, Non-Patent Document 8); and imidazopyridine derivatives such as 6-iodo-2-[4′-(N,N-dimethylamino)phenyl]imidazo[1,2-a]pyridine (hereinafter referred to as IMPY) (Patent Document 3, Non-Patent Document 9). Further, some of these probes for imaging diagnosis have been studied on human imaging and have been reported to show a significant accumulation in AD patient's brain compared with normal persons (Non-Patent Document 10, Non-Patent Document 11).    [Patent Document 1] JP-T-2004-506723    [Patent Document 2] JP-T-2005-504055    [Patent Document 3] JP-T-2005-512945    [Patent Document 4] JP-T-2002-523383    [Non-Patent Document 1] J. A. Hardy & G. A. Higgins, “Alzheimer's Disease: The Amyloid Cascade Hypothesis.”, Science, 1992, 256, p. 184-185    [Non-Patent Document 2] G. McKhann et al., “Clinical diagnosis of Alzheimer's disease: Report of the NINCDS-ADRDA Work Group under the auspices of Department of Health and Human Services Task Force on Alzheimer's Disease.”, Neurology, 1984, 34, p. 939-944    [Non-Patent Document 3] Z.-P. Zhuang et al., “Radioiodinated Styrylbenzenes and Thioflavins as Probes for Amyloid Aggregates.”, J. Med. Chem., 2001, 44, p. 1905-1914    [Non-Patent Document 4] Masahiro Ono et al., “11C-labeled stilbene derivatives as Aβ-aggregate-specific PET imaging agents for Alzheimer's disease.”, Nuclear Medicine and Biology, 2003, 30, p. 565-571    [Non-Patent Document 5] H. F. Kung et al., “Novel Stilbenes as Probes for amyloid plaques.”, J. American Chemical Society, 2001, 123, p. 12740-12741    [Non-Patent Document 6] Zhi-Ping Zhuang et al., “IBOX(2-(4′-dimethylaminophenyl)-6-iodobensoxazole): a ligand for imaging amyloid plaques in the brain.”, Nuclear Medicine and Biology, 2001, 28, p. 887-894    [Non-Patent Document 7] Furumoto Y et al., “[11C]BF-227: A New 11C-Labeled 2-Ethenylbenzoxazole Derivative for Amyloid-β Plaques Imaging.”, European Journal of Nuclear Medicine and Molecular Imaging, 2005, 32, Sup. 1, P 759    [Non-Patent Document 8] Eric D. Agdeppa et al., “2-Dialkylamino-6-Acylmalononitrile Substituted Naphthalenes (DDNP Analogs): Novel Diagnostic and Therapeutic Tools in Alzheimer's Disease.”, Molecular Imaging and Biology, 2003, 5, p. 404-417    [Non-Patent Document 9]-Zhi-Ping Zhuang et al., “Structure-Activity Relationship of Imidazo[1,2-a]pyridines as Ligands for Detecting β-Amyloid Plaques in the Brain.”, J. Med. Chem, 2003, 46, p. 237-243    [Non-Patent Document 10] W. E. Klunk et al., “Imaging brain amyloid in Alzheimer's disease with Pittsburgh Compound-B.”, Ann. Neurol., 2004, 55, p. 306-319    [Non-Patent Document 11] Nicolaas P. L. G. Verhoeff et al., “In-Vivo Imaging of Alzheimer Disease β-Amyloid With [11C]SB-13 PET.”, American Journal of Geriatric Psychiatry, 2004, 12, p. 584-595